I didn't really want to admit this, but many years ago I did the same, but without the need to manually agitate the beans. I put a switch in between the heating element and the power connector so I could turn on and off the power just to the element. I found it very difficult to get reproducible results with this and quickly binned it. If you do want to try it you may find it isn't even necessary to cut any wires. On the popper I had the heater was attached by normal spade connectors so I could just put the same connectors on the switch and insert it in between the terminals. It will still be necessary to drill a hole in the housing for the switch, or alternatively put the switch in a separate housing of some kind.
I hope that it goes without saying that the mains plug must be pulled before opening up the machine or making any modifications to it.